The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with medical magnetic resonance imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention also finds application in conjunction with other types of magnetic resonance imaging systems, magnetic resonance spectroscopy systems, and the like.
In magnetic resonance imaging, linear magnetic field gradients are used for spatial encoding. Gradient coils are used to produce the linear magnetic field gradients. Gradient coils are generally designed to provide an imaging field-of-view (FOV) which is fixed in size. For example, in whole-body applications the gradient coil will typically be designed to produce sufficiently linear or uniform magnetic field gradients over a 45-50 cm diameter spherical volume (DSV). For a dedicated cardiac scanner, however, the DSV may be 35-40 cm. For a dedicated head system the linear gradients would be designed to produce sufficiently linear magnetic field gradients over a 20-25 cm DSV. As the useful DSV is made smaller the stored energy of the gradient coil is reduced, which allows for higher performance, namely, higher peak gradient strengths and faster gradient coil switching. Outside the substantially linear region of the gradient field (i.e., the xe2x80x9cusefulxe2x80x9d DSV), and to a lesser extent within, the magnetic field gradients produce image distortion. Software-based distortion correction schemes have been developed to correct for nonuniformities within the useful DSV, as well as to expand somewhat the useful imaging FOV.
In each dedicated case noted above, the gradient coil is generally a unique electromechanical structure and gradient coils with a defined DSV are known and utilized throughout the industry. For example, the most common is a self-shielded symmetric gradient coil design for whole-body imaging applications. Dedicated head and cardiac/head coil designs have emerged to enhance performance (peak strength and switching rate) over a reduced imaging DSV. Generally, body access is desirable for patient comfort reasons, although dedicated head gradient designs continue to be discussed for advanced neuro/brain research applications.
Gradient coils are heavy electromechanical devices, unlike an RF surface coil which can be easily removed and replaced with a different RF surface coil between imaging procedures. A gradient coil, due to its high power nature, tends to be a fixed entity within an MRI system. As such, a dedicated gradient coil tends to make the MRI system a dedicated imaging system, limiting its scope of clinical application. Thus, accommodating both large and small FOV applications has generally required either separate dedicated machines, which is expensive, or the use of dedicated insertable coils for the smaller volumes, which are heavy and difficult to insert or replace.
More recently, dual or twin gradient designs have been described in the literature which attempt to combine both large volume and high-performance small volume imaging capabilities into a single gradient coil electromechanical package. Katznelson et al., in U.S. Pat. No. 5,736,858, describe a means for providing two gradient coils which can be configured to allow for two different useful DSVs. Each gradient axis, x, y, and z, has two gradient coils. One gradient coil is designed to produce a linear magnetic field gradient over a first DSV, and a second gradient coil is designed to produce a second linear magnetic field gradient, such that when the second gradient coil is driven in series with the first gradient coil, there results a second DSV that is larger than the first DSV. In this scheme, the DSV can take two discrete values but it is not continuously variable. The first gradient coil has lower stored energy and can be switched faster than the second gradient coil alone or when the two gradient coils are connected in series. In another embodiment, the first coil produces a gradient for use in small FOV applications and the second coil produces a gradient for use in conventional, large FOV applications and a single amplifier means and a switching means allows for one or the other coil to be used separately. In the preferred embodiment, the first coil is used for fast-switching, small FOV imaging and both coils together are used for larger FOV imaging and/or to produce very high gradient strengths, which may find use in diffusion imaging applications. A key point is that each coil is designed so as to produce, alone or in combination, a linear gradient magnetic field over one of two possible imaging DSVs. In the preferred embodiment the two coils are used together (in series) to produce a relatively large DSV. In the alternate embodiment, each coil can be used individually to create reasonably non-distorted magnetic resonance images over two differently sized DSVs. Katznelson et al. do not teach that the second gradient coil be preferentially configured/designed to produce substantially zero first-order gradient. Furthermore, each coil is self-shielded or actively-shielded in design to minimize eddy current effects. Since each coil produces its own linear gradient magnetic field, a drawback of this approach is that two full-power gradient coils are housed within one electromechanical assembly. This consumes a great deal of radial space, particularly when the two coils occupy different radial positions within the electromechanical structure. Each coil must have sufficient conductor cross-section to carry similar high currents. Also, cooling of the two coils becomes an issue, as does the ability to fit in other components such as passive and electrical shim coils.
It has also been proposed by Kimmlingen et al. (xe2x80x9cGradient system with continuously variable field characteristics,xe2x80x9d ISMRM 2000 (April, 2000, Denver meeting)) to take a standard whole body coil with a large field of view and identify a subset of the coil windings which would produce a linear gradient in a smaller FOV, but with comparable (about 20% less) peak gradient strength and substantially lower inductance (about 45% less), allowing for faster gradient switching. Switching means or a dual amplifier design, to feed both coil sections separately, would be provided such that either the subset or all of the windings could be utilized, and the amount of current to subset or other windings could be adjustable, depending on the size of the FOV. The primary advantage is that the two coils occupy the same radial position with the normal six layers, making cooling and construction easier and more cost-effective. A disadvantage of this approach is that when some of the coil windings were taken away to provide for the smaller FOV, some gradient strength was lost. Another disadvantage is that shielding was compromised since only the combined coils were optimally shielded, leading to increased eddy current effects.
Petropoulos, in U.S. Pat. No. 6,049,207, describes a dual gradient coil assembly with two primary coils and one common shield coil. Each primary coil produces a linear magnetic field gradient over differently sized DSVs when operated with the common shield coil. The residual eddy current effects are not equal for the two coils, one inevitably is better than the other. However, this is minimized by constraining each continuous current primary coil and common shield coil combination to have an integer number of turns before discretization. The approach of having one common shield does save some radial space for manufacture. However, two high power primary coils are still required.
It is also known in the art to use relatively low current/power shim coils to improve the uniformity of the main magnetic field. However, such shim coils are generally designed to produce predominantly one order of correction, for example, a Z2 shim coil provides primarily a second-order correction field, a physically separate Z3 shim coil a third-order correction field, and so on.
The present invention contemplates a new and improved gradient coil system which provides a variable imaging field of view, and which overcomes the above referenced problems and others.
In a first aspect of the present invention, a magnetic resonance imaging apparatus includes a main magnet system for generating a main magnetic field through an examination region, a radio frequency coil disposed adjacent the examination region for transmitting radio frequency signals into the examination region and selectively exciting dipoles disposed therein, and a radio frequency transmitter for driving the radio frequency coil. A receiver receives magnetic resonance signals from resonating dipoles within the examination region and an image processor reconstructs an image representation from the received magnetic resonance signals for display on a human readable display.
The apparatus further includes a gradient coil assembly for generating magnetic field gradients across the main magnetic field. The gradient coil assembly includes a base gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates magnetic field gradients which are substantially linear over a first useful imaging volume. The gradient coil assembly further includes a correction gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates magnetic field gradients having substantially no first order moment. The correction gradient coil set produces third and higher order moments which combine with higher order terms of the base gradient coil set to produce magnetic field gradients which are substantially linear over a second useful imaging volume which is different from the first useful imaging volume when the correction gradient coil set is used in combination with the base gradient coil set.
In another aspect, a gradient coil assembly for inducing magnetic field gradients across an examination region in a magnetic resonance imaging apparatus is provided. The gradient coil assembly includes a base gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates the substantially linear magnetic field gradients which are substantially linear over a first imaging volume and nonlinear outside of the first imaging volume, and a correction gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates a magnetic field gradient having third and higher order terms and substantially no first order term, the third and higher order terms combining with higher-order terms of the base gradient coil set to produce magnetic field gradients which supplement the nonlinear gradient surrounding the first imaging volume to increase the imaging volume over which the gradient is linear from the first imaging volume to a second imaging volume.
In still a further aspect, a method of magnetic resonance imaging comprises: generating a temporally constant magnetic field through an examination region of a magnetic resonance imaging apparatus, exciting and manipulating magnetic resonance in selected dipoles in the examination region, demodulating magnetic resonance signals received from the examination region, reconstructing the demodulated resonance signals into an image, and, in appropriate time sequence to the above actions, inducing gradient magnetic fields across the temporally constant magnetic field with a gradient coil assembly. The gradient coil assembly comprises a base gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates the substantially linear magnetic field gradients defining a first useful imaging volume, and a correction gradient coil set disposed about the examination region including an array of conductive coil loops arranged such that a current density flowing thereon generates substantially no linear magnetic gradient. The correction gradient coil set produces third and higher order and substantially no first order gradient. The third and higher order gradients combine with higher-order terms of the base gradient coil set to produce the substantially linear magnetic field gradients defining a second useful imaging volume which is different from the first useful imaging volume when the correction gradient coil set is used in combination with the base gradient coil set.
In yet a further aspect, in a method of magnetic resonance imaging, a method of producing a magnetic field gradient which is generally linear over a selected imaging volume comprises configuring a base gradient coil to produce a first magnetic field gradient in response to supplying a first current to the base gradient coil, the first magnetic field gradient being generally linear over a first useful imaging volume, the first magnetic field gradient increasing proportionally with the supplied current, and configuring a correction coil to produce a second magnetic field gradient in response to supplying a second current to the correction coil, the second magnetic field gradient having substantially no first order moment, the second magnetic field gradient combining with the first magnetic field gradient to generate a combined magnetic field gradient which is generally linear over a second useful imaging volume.
In still another aspect, a method of designing a gradient coil system for a magnetic resonance imaging system having a variable useful imaging diameter spherical volume, comprises designing a base coil that produces a first magnetic field gradient that is generally linear over a first imaging volume, and designing a correction coil that produces a second magnetic field gradient having substantially zero first order moment, the first and second magnetic field gradients combining to produce a third magnetic field gradient that is generally linear over a second imaging volume that is different from the first imaging volume.
It will be recognized that a coil with zero first order gradient represents the ideal case and the term xe2x80x9csubstantially zero first orderxe2x80x9d as used herein is not intended to preclude a minimal first order gradient. Likewise, the term xe2x80x9csubstantially linearxe2x80x9d as used herein is not intended to preclude small nonlinearities or nonuniformities in the gradient fields.
One advantage of the present invention is that gradient fields with linear regions of variable spatial extent can be generated to accommodate both large volume imaging applications and small volume imaging applications requiring fast gradient coil switching and high peak gradient strengths.
Another advantage is that the linear region of the gradients can be tailored to the region of interest, thus reducing the potential for peripheral nerve stimulation in an examination subject.
Another advantage is that the variable field of view is accomplished by providing a correction coil which generates a substantially zero first order gradient field and which therefore carries much less current than would be required for a second coil producing a linear gradient field.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.